I've been investigating this today... Certainly this tethering in general works in vanilla OpenWRT (which pineapple is based on). I'm coming across multiple road-blocks in finding the information readily (I'm a bit of a newb) but from what I understand, once the usb device is detected and using the appropriate driver, usb0 will become an available network interface.
What I can't seem to get working is loading the usb-net drivers - based on other forums I've read, the following lines should show in the dmesg/logread prior (but are missing for me):
usbcore: registered new interface driver cdc_ether
usbcore: registered new interface driver rndis_host
I've install the opkg packages for cdc and rndis, but these do not seem to load as drivers for me. I'm running Pineapple MK4 1.1.1 + opkgmanager. If I make any progress, I'll post it here...
ref: https://forum.openwrt.org/viewtopic.php?pid=146421